Nylon is a synthetic polymer, invented by Wallace Carothers of the E.I. du Pont de Nemours and Company in 1935. The first product, a nylon bristle toothbrush made went on sale in 1938. Nowadays, the most commonly used nylon are nylon 66, also called nylon 66 (polyhexamethylenediamine adipamide), originally patented by DuPont, and nylon 6 (polycaprolactamide) produced by IG Farben.
Nylon 6 polymer is produced from caprolactam. Caprolactam is derived most commonly from cyclohexanone, which in turn comes from either phenol or cyclohexane. About 70% of all nylon 6 polymer is produced by continuous polymerization. Nylon 66 polymer is made from adipic acid and hexamethylene diamine, which react to form hexamethylene diammonium adipate (AH salt). Polymerization then takes place under heat and pressure in a batch process. The fiber spinning and processing procedures are achieved by using a process called melt spinning. Melt spinning uses heat to melt the polymer to a viscosity suitable for extrusion. This type of spinning is used for polymers that are not decomposed or degraded by the temperatures necessary for extrusion. Polymer chips may be melted by a number of methods. The trend is toward melting and immediate extrusion of the polymer chips in an electrically heated screw extruder. Alternatively, the molten polymer is processed in an inert gas atmosphere, usually nitrogen, and is metered through a precisely machined gear pump to a filter assembly consisting of a series of metal gauges interspersed in layers of graded sand or metal powder. The molten polymer is extruded at high pressure and constant rate through a spinnerette into a relatively cooler air stream that solidifies the filaments. Lubricants and finishing oils are applied to the fibers in the spin cell. At the base of the spin cell, a thread guide converges the individual filaments to produce a continuous filament yarn, or a spun yarn, that typically is composed of between 15 and 100 filaments. Once formed, the filament yarn is immediately wound onto bobbins, or it is further treated for certain desired characteristics or for end use (texturisation). Polyamide yarns made from Nylon 6, 66, 69 and 610 can be used in the textile industry in both knitting and weaving with high efficiency to form high quality and fashionable garments. These polymers, especially Nylon 6 and 66 are used in the production of knitted leg-wear and body-wear garment. In these products, dyeing efficiency and cost effective dyeing processes are important considerations.
Total world demand for nylon resins reached 2.3 million metric tons in 2006 (E. V. Ormonde and H. Mori, “Nylon Resins”, CEH Report, 2007). Total world consumption of nylon resins is forecast to increase to about 2.8 million metric tons by 2011, representing an average annual growth rate of 4%. Reasonably good growth is projected to have resumed in major markets such as automotive parts, industrial/machinery, electrical/electronics and film in 2007. In the United States, nylon 66 is the major type of nylon resin produced, in part because of DuPont's major position in the production of this polymer. In Japan, nylon 6 is the major resin type. In Western Europe, however, the market share of nylon 66 is about equal to that of nylon 6. Other nylons (e.g., nylons 11 and 12) are considered specialty products and are consumed in much smaller quantities
Nowadays, numerous efforts have been made toward environmentally responsible methods of manufacturing nylon, including finding ways to use waste products from the manufacture of various synthetic materials (post-industrial waste) and recycling waste from synthetic products (post-consumer waste). Typical post-industrial waste synthetic polymeric product are leftovers from the different manufacturing steps, such as extrusion or molding of a polymeric product from virgin polymer. For instance, when nylon is spun into yarn, there is a leftover fiberstock that is off-specification yarn that would be disposed by the manufacturer. Such leftover can be treated to form flakes, chips or pellets, and it can be recycled in the process of the present invention. Other kind of recovery processes are monomer recovery systems, which are especially used on caprolactam volatilized at the spinnerette during nylon 6 fiber formation. Monomer recovery systems are not used in nylon 66 spinning operations, because nylon 66 does not usually contain a significant amount of residual monomer. Recycling method for post-consumer waste of nylon 6 based material is described for example in U.S. Pat. No. 6,187,917. Concerning the treatment of nylon 66, most of the methods known in the prior art are directed towards recycling post-consumer waste, e.g. synthetic carpets, in order to reintroduce the gathered polymer into recycling processes. The same methods are generally applied for recycling post-industrial waste, as described in the following publications:
U.S. Pat. No. 7,319,113 relates to a process for recovering polyamide material from post-industrial and post-consumer products containing a polyamide material. Said process includes contacting the post-industrial and post-consumer products with a suitable solvent in a reactor; dissolving and partially depolymerizing the polyamide material in the solvent to form a solution; separating the insoluble material from the solution; and recovering the depolymerized polyamide from the separated solution.
US 2004/0249001 relates to a process for the solution recovery of nylons from various post-industrial and post-consumer products. Said process includes contacting the waste products with hexamethylene diamine and in a suitable solvent in a reactor; dissolving and depolymerizing the polyamide material; separating insoluble materials from the solution; recovering of the depolymerized polyamide from the separated solution; and repolymerizing the depolymerized polyamide.
US 2004/0053047 relates to a process for making colorable mono-component filament. Said process comprises: blending virgin polymer A and a recycled polymer A to form a mixture; and forming the mixture into colorable mono-component filaments that have good colorability characteristics, comparable to those of colorable mono-component filament formed from the virgin polymer A free of the recycled polymer A. However, the described process lacks a monitoring system which may control in real-time the quality of the filaments produced and readjust, if necessary, the virgin polymer/recycled polymer ratio. This is an essential feature to guarantee the quality and homogeneity of the filaments produced in a continuous recycling process, where the source and quality of the recycled polymer may vary.
Most of the above-mentioned methods involve numerous complicated steps, including depolymerization and repolymerization of the polymer in various solvents, and several steps of purification/separation before obtaining the recycle polyamide. There is therefore a need for a simple method to obtain recycled polyamide yarns from waste polymer which could overcome the disadvantages of the prior art.
It is therefore an object of the present invention to provide a method for the production of polyamide yarns, more friendly to the preservation of the environment than known methods.
It is further an object of the present invention to provide a convenient, low cost, and power saving method for the production of recycled polyamide yarns with polyamide from post-industrial waste.
It is another object of the present invention to provide a method for the production of recycled polyamide yarns with a high waste polymer content.
It is still another object of the present invention to provide a method for the continuous production of recycled polyamide yarns including a monitoring step for checking the quality of the produced yarns and readjusting in real time, if necessary, the composition of the initial blend mixture.
It is a still further object of the present invention to provide a method for the production of recycled polyamide yarns having improved elongation capacity and a dyeability similar to non-recycled yarns.
Other objects and advantages of the present invention will appear as the description proceeds.